MILWAUKEE – Wisconsin's heroin problem continues to grow because traffickers have figured out how to meet the demand for a cheaper alternative to prescription drugs with an inexpensive but highly pure product, the FBI said Tuesday.

Law enforcement officials have seen a spike in heroin usage since about 2008, but the actual scope of the problem is hard to quantify because of differences in how usage is reported throughout the state.

So the FBI and other agencies conducted a yearlong analysis using whatever limited information was available, the agency told The Associated Press. The result is a new report called the Wisconsin Heroin Assessment, the largest effort to date to gauge how bad the problem is and how much worse it could get.

Among its findings:

— The drug mainly enters the U.S. from South America and Mexico and finds its way to Wisconsin by way of Minneapolis, Chicago, and Rockford, Illinois. Because heroin is transported in small, concentrated amounts, it is harder for authorities to detect.

— Users are typically white males between the ages of 21 and 25.

— Users generally start out abusing pills such as Vicodin and Percocet, which are opiates like heroin. They tend to switch to heroin as a cheaper, more potent alternative.

— One hit (usually one-tenth of a gram) costs $12 to $15 in Milwaukee and about twice that — due to supply and demand — in the Green Bay area.

"Heroin abuse in Wisconsin will continue to be a threat. It shows no signs of abating," said John Kumm, an intelligence analyst in the FBI's Milwaukee office.

The number of Wisconsin deaths due to heroin overdose has risen steadily over the years, according to the state Department of Health Services. There were 187 deaths in 2012, the most recent year for which data are available. That's nearly triple the 67 people who died of heroin overdose in 2008.

The figures may not be complete, because there's no statewide standard for tracking heroin deaths. As a result, the statistics only count people whose death certificates specifically list heroin abuse as the cause of death, but not if it says something more generic such as 'drug abuse.'

Dana Brueck, a spokeswoman for the state Department of Justice, said several state agencies compile their own statistics on drug abuse. Because the issue spans multiple agencies, she said, it would require a state law to standardize reporting and set up a tracking system.

Kumm (pronounced 'koom') said Wisconsin is the only state he's aware of that has conducted such a detailed statewide assessment. He said he hopes other states follow Wisconsin's lead, to help develop a more comprehensive national snapshot.

Heroin was a major problem in the U.S. in the 1970s. Users migrated to cocaine in the '80s and crack cocaine in the '90s, Kumm said, but as prescription opiates became more easily available in recent years, drug users went to pills and eventually back to heroin.

The problem is so difficult to overcome because the drug is terribly addictive, he said. A high lasts for hours, perhaps up to a day, but users seek a new fix as much to avoid horrible withdrawal symptoms as to get another high. The pattern becomes pervasive, and users can struggle with addiction for a lifetime.

The report was a joint collaboration between the FBI, U.S. Drug Enforcement Agency and three state anti-drug agencies.